These days, wireless communication systems using wireless communication channels for purposes of communication, for example, wireless communication systems based on WCDMA, CDMA2000 and the like are in widespread use on a global scale. In such wireless communication systems, a plurality of wireless base stations are installed and a wireless terminal device communicates with another communication device, for example, a communication terminal device via any of the wireless base stations. The service area of a wireless base station within which the wireless base station can communicate with wireless terminal devices has an overlap with the service area of a neighboring wireless base station so that when the wireless channel condition deteriorates, handover to the neighboring wireless base station may be executed.
Meanwhile, wireless technologies commonly used include code-division multiplexing, time-division multiplexing, frequency-division multiplexing, and OFDM (Orthogonal Frequency-Division Multiplexing), for example, which all enable a plurality of wireless terminal devices to simultaneously connect to a single wireless base station.
However, even inside the service area of a wireless base station where wireless communication is available, it is often difficult for a wireless terminal device to make use of high-speed communication because of poor wireless environment if the wireless terminal device is located near the boundary of the service area. Also, because of factors that obstruct the propagation of wireless signals such as buildings, an area in which the wireless terminal device has difficulty in establishing a satisfactory wireless connection with a wireless base station, that is, a so-called dead zone, can be created even at a location well inside the service area. In the circumstances, a method has been proposed in which a relay station (hereinafter referred to as RS) is installed within the service area of a wireless base station so that a wireless terminal device and the wireless base station can wirelessly communicate with each other via the RS.
Also, in connection with wireless communication systems using the OFDM technology and complying with the IEEE 802.16, specifications of relay stations have been defined by the IEEE 802.16j Task Group.
FIG. 17 is a diagram explaining an exemplary flow of automatic repeat request (ARQ) control according to the IEEE 802.16j. Specifically, FIG. 17 exemplifies End-to-End mode in which ARQ is executed at a wireless base station (hereinafter referred to as BS) and a wireless terminal device (hereinafter referred to as MS).
A PDU (Protocol Data Unit), which is a transmission unit for packets, includes fixed blocks each assigned a BSN (Block Sequence Number). In the example illustrated in FIG. 17, the PDU transmitted from the BS has a size corresponding to five fixed blocks. If the fixed block at the beginning of the PDU has the BSN “5”, namely, BSN=5, the BSN of the fixed block at the tail of the PDU is “9”. Also, the PDU is transmitted with the BSN of the first fixed block included in its header information. In the example of FIG. 17, the BSN “5” is included in the header information.
The RS receives the PDU from the BS and, if the wireless channel condition is poor, divides the PDU for transmission to the MS, in which case each divided PDU is transmitted with the BSN of the fixed block at the beginning thereof included in the header information. In the example of FIG. 17, the BSNs and “7” are included in the header information of the respective two divided PDUs. When a PDU is divided at the RS, the RS divides the PDU into groups of fixed blocks.
The MS notifies the BS whether the divided PDUs have been successfully received from the RS or not by sending information indicative of success or failure of reception, namely, Acknowledgement (ACK) information or Not Acknowledgement (NAK) information. In the example illustrated in FIG. 17, the divided PDU having the BSN “5” included in its header information, out of the two divided PDUs, fails to be received. Accordingly, the MS transmits, to the BS, NAK with respect to the BSNs “5” and “6” and ACK with respect to the BSNs “7” to “9”. When notified of the NAK and the ACK, the BS retransmits the fixed blocks corresponding to the BSNs “5” and “6”. In this manner, the information that failed to be received by the MS, in the example of FIG. 17, the divided PDU including the fixed blocks with the BSNs “5” and “6” is transmitted from the BS to the MS via the RS. The foregoing related to the IEEE 802.16 is described, for example, in IEEE Std 802.16-2009 and IEEE Std 802.16j-2009 identified below.
Also, a relay station and a relay method have been known which are capable of preventing conflict of FSN (Fragment Sequence Number) from occurring when a packet is divided by an IEEE 802.16j-compliant relay station. Such relay station and method are disclosed, for example, in International Publication Pamphlet No. WO 2008/078365 identified below.
International Publication Pamphlet No. WO 2008/078365
IEEE Std 802.16-2009
IEEE Std 802.16j-2009
The IEEE 802.16m Task Group has been newly launched and is currently in the process of developing and investigating specifications for next-generation wireless links. The ongoing development involves development of relay stations complying with the specifications for the new wireless links.
With the IEEE 802.16m, the ARQ process can be performed on PDUs of variable block size. That is, the size of the PDU used in the IEEE 802.16m is variable, and the BS transmits each PDU with a sequence number (SN) affixed thereto as a packet number. The MS generates ACK or NAK indicative of success or failure of reception of the PDU, and transmits the generated ACK or NAK to the BS via the RS. The ACK or NAK thus transmitted includes an SN whereby the PDU can be identified.
As regards the specifications for the next-generation wireless links wherein the ARQ process is performed on PDUs of variable block size, however, no proposal has been made so far as to how to generate divided PDUs to be transmitted from a relay station to a receiving station and how to smoothly carry out the ARQ process.